Many electrical, electronic, and optoelectronic devices employ an electrode covering some portion of a generally two-dimensional surface of the device through which light enters or exits during the ordinary and intended operation of the device. Ideally, the electrode provides a means of making perfect electrical connection to the device while not inhibiting the transmission of light either into or out of the device.
For example, light incident on the light-receiving side of a photovoltaic device must be allowed to pass to the device's p-n junction to generate current. To carry out its intended function, the device in turn must be connected through an electrode structure to an external load that is appointed to be energized.
Other devices emit light in their expected and ordinary function. For example, input electrical energy is converted to light by a semiconductor or similar structure that is an integral part of devices such organic light-emitting devices or displays (OLEDs), touch screen panels, flat-panel displays, and various forms of solid state lighting. The light must be produced and delivered efficiently to the external world.
In most such devices, the incident or emitted light must pass through a substantial part of a surface of the device, which is nominally planar and two-dimensional, but may also be curved to some degree. Typically, the electrode structure must make contact throughout this area. A thick metallization layer covering most or all of a device's surface may afford high conductivity, but it provides no transparency through the covered area. Therefore, other expedients are essential. One approach relies on a high conductivity, thick material that covers only a small portion of the functional surface. Alternatively, most or all of the functional surface may be covered with a material that is inherently conductive but also transparent.
For example, a common electrode structure for the front (light-receiving) side of a photovoltaic cell is made by screen-printing a paste containing metallic silver particles in a pattern of thin, connected traces. The paste ordinarily is fired to sinter the silver particles. A typical configuration features a comb-like structure comprising a central large bus bar, with a large number of thin traces that branch off. The traces may be 10-20 μm thick and 50-150 μm wide, and are fully opaque. The currents collected by the individual traces join and flow through the bus bar, which is appointed in turn to be connected to one side of an external electric load. The inherently high conductivity of the silver used in the traces promotes a high conductivity, which desirably reduces the effective source resistance of the cell and the resulting ohmic loss produced by current flowing from the cell. However, the traces and bus bar cover a finite but measurable part of the overall cell area, thereby reducing the efficiency of the cell, since the shadowed areas cannot generate photoelectricity.
While extremely thin layers of metals do transmit light to some degree, their conductance at such a thickness is too low for virtually all practical optoelectronic devices. Thus, an alternate approach is taken for devices such as OLEDs in touch screen panels, flat-panel displays, and solid state lighting. Ordinarily, the configuration of these devices demands a uniform conducting layer that cannot be satisfied with a patterned metallization such as that described above for ordinary photovoltaic cells.
Heretofore, a conventional expedient has been the use of transparent conductive oxides, most commonly indium-tin-oxide (ITO), although aluminum-doped zinc oxide (AZO) and fluorine-doped tin oxide (FTO) are also possible. These oxides can be applied over the whole surface of a device. For a suitably chosen thickness (often in the range of 100 nm-300 nm), they have conductivity and transparency that are workable for many applications. However, depositing ITO requires a complicated fabrication with expensive equipment. Over time, indium can migrate into adjacent active material, potentially degrading performance. Furthermore, indium is a relatively rare and expensive element.
Another detriment is that ITO and similar conductive oxides still have conductivities that are substantially lower than those of highly conducting metals like Ag, Cu, and Al. In principle, increasing the thickness of the oxide layer should increase sheet conductance (or equivalently, decrease the sheet resistance). But in typical practice, the expected enhancement in sheet conductance is not realized, because of microcracking and other like microstructural defects. The same mechanical phenomena are also manifested in the materials' brittleness and inflexibility, impeding their use in flexible devices which are increasingly desired. Increased thickness also leads to an undesirable degradation of the layer's optical transparency.
Consequently, there remains a need in the art for transparent, conductive materials and structures that may be employed in the construction of reliable, efficient optoelectronic devices.